In organic chemistry, an alkene is an unsaturated hydrocarbon that contains at least one carbon–carbon double bond. The words alkene and olefin are often used interchangeably.

1. Alkenes: Reactions and
Synthesis
Md. Saiful Islam
Dept. of Pharmaceutical Sciences
North South University
Facebook Group: Pharmacy Universe
Md. Saiful Islam
Dept. of Pharmaceutical Sciences
North South University
Facebook Group: Pharmacy Universe

2. Diverse Reactions of Alkenes
• Alkenes react with many electrophiles to give useful products by
addition (often through special reagents)
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3. Preparation of Alkenes: A Preview of
Elimination Reactions
• Alkenes are commonly made by
– elimination of HX from alkyl halide (dehydrohalogenation)
• Uses heat and KOH
– elimination of H-OH from an alcohol (dehydration)
• require strong acids (sulfuric acid, 50 ºC)
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4. Addition of Halogens to Alkenes
• Bromine and chlorine add to alkenes to give 1,2-dihaldes, an
industrially important process
– F2 is too reactive and I2 does not add
• Cl2 reacts as Cl+
Cl-
– Br2 is similar
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5. Addition of Br2 to Cyclopentene
• Addition is exclusively trans
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6. Mechanism of Bromine Addition
• Br+
adds to an alkene producing a cyclic ion
• Bromonium ion, bromine shares charge with carbon
– Gives trans addition
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7. Bromonium Ion Mechanism
• Electrophilic addition of bromine to give a cation is followed by cyclization
to give a bromonium ion
• This bromonium ion is a reactive electrophile and bromide ion is a good
nucleophile
• Stereospecific anti addition
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8. Addition of Hypohalous Acids to
Alkenes: Halohydrin Formation
• This is formally the addition of HO-X to an alkene to give a 1,2-
halo alcohol, called a halohydrin
• The actual reagent is the dihalogen (Br2 or Cl2 in water in an
organic solvent)
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9. Mechanism of Formation
of a Bromohydrin
• Br2 forms bromonium ion, then water adds
– Orientation toward stable C+
species
– Aromatic rings do not react
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10. Addition of Water to Alkenes:
Oxymercuration
• Hydration of an alkene is the addition of H-OH to to
give an alcohol
• Acid catalysts are used in high temperature industrial
processes: ethylene is converted to ethanol
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11. Oxymercuration Intermediates
• For laboratory-scale hydration of an alkene
• Use mercuric acetate in THF followed by sodium borohydride
• Markovnikov orientation
– via mercurinium ion
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12. Reduction of Alkenes: Hydrogenation
• Addition of H-H across C=C
• Reduction in general is addition of H2or its equivalent, or a loss of O
from the molecule
• Requires Pt or Pd as powders on carbon and H2
• Hydrogen is first adsorbed on catalyst
• Reaction is heterogeneous (process is not in solution)
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13. Catalytic
Hydrogenation
Mechanism
• Heterogeneous –
reaction between phases
• Addition of H-H is syn
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14. Permanganate Oxidation of Alkenes
• Oxidizing reagents other than ozone also cleave alkenes
• Potassium permanganate (KMnO4) can produce carboxylic acids
and carbon dioxide if H’s are present on C=C
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15. Free Radical Polymerization: Initiation
• Initiation - a few radicals are generated by the reaction of a molecule
that readily forms radicals from a nonradical molecule
• A bond is broken homolytically
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16. Polymerization: Propagation
• Radical from initiation adds to alkene to generate alkene radical
• This radical adds to another alkene, and so on many times
• Chain propagation ends when two radical chains combine
• Not controlled specifically but affected by reactivity and concentration
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Termination

17. Other Polymers
• Other alkenes give other common polymers
• Radical stability: 3o
> 2o
> 1o
(just like with carbocations)
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